1. Field of Invention
The invention relates to a capacitive sensor with at least one reference impedance and at least one measuring condenser, with at least one electrical alternating signal source, with a current supply network, as well as with an analysis unit, whereby the reference impedance and the measuring condenser are connected via the current supply network to the alternating signal source and the analysis unit such that the charge and discharge currents of the reference impedance and the measuring condenser can be analyzed by the analysis unit.
2. Description of Related Art
Capacitive sensors of the initially mentioned type are known from, for example, the U.S. Pat. Nos. 5,650,730 and 5,793,217 and are used for determining the capacitance of the measuring condenser or the change in the capacitance of the measuring condenser. These sensors have a special current supply network, namely such a one that consists of diodes, connected in series, which form a diode ring.
In the capacitive sensor itself, often only one electrode of the measuring condenser is designed, and the other electrode of the measuring condenser is formed by the surrounding area of the capacitive sensor. The measuring condenser is thus normally not a condenser in terms of a complete electrotechnical component, but rather an arrangement that is equipped with a capacitance whose active electrode is assigned to the capacitive sensor, whereby an electrical stray field extends from the active electrode into the surrounding area.
In the prior art, in most cases, a reference condenser is used as a reference impedance. Where there is concrete mention below of a reference capacitance, the embodiments still generally also apply to a reference impedance; the capacitance of the reference condenser then corresponds to the value of the reference impedance, independently thereof, as the reference impedance is implemented as a component. The charge and discharge currents of a reference condenser then correspond to the charge and discharge currents of a reference impedance, whereby the reference impedance can convert the energy supplied to it in some other way than only in the electric field of a condenser.
The capacitance of the above-described sensor can be changed, on the one hand, if the geometry of the arrangement, and thus, the stray field of the active electrode is changed; on the other hand, the capacitance of the sensor—without a change in the extension of the stray field—can also change in an alteration of the dielectric properties of the space, in which the electric field extends. Because of these general properties, capacitive sensors are frequently used as proximity switches and as fill-level detectors.
In capacitive sensors of the initially described type, the alternating signal source is usually designed as an oscillator, such as, for example, as a harmonic oscillator in the form of an LRC network, which is switched in such a way that it executes a continuous oscillation. As the signal level within the positive semioscillation of the alternating signal increases, the measuring condenser is charged via a current that flows over a first path of the diode ring, and the reference impedance—frequently designed as a reference condenser—is charged during the latter with a current that flows over a second path of the diode ring. The charge current that flows over one path in each case is derived as a discharge current via the respective other path.
The mode of operation of the above-described capacitive sensor is consequently based on the fact that the charge current of the reference impedance, which can be configured in particular as a reference condenser, or the charge current of the measuring condenser in each case flows over a path of the sensor or the current supply network that is different from the discharge current of the reference condenser or the measuring condenser. If the capacitances of the reference condenser and the measuring condenser are equally large, the current that flows in via the first path or the second path on average is equal to the current that flows out from the first path or from the second path of the current supply network that is designed as a diode ring. If the capacitances of the reference condenser and the measuring condenser vary in size, however, a resulting current is produced in the time mean in each path of the current supply network that is designed as a diode ring. By analyzing the differential currents of the charge and discharge currents that flow into the first and second paths of the current supply network, it is evident what the ratio is between the capacitance of the measuring condenser and the capacitance of the reference condenser.
In the generic capacitive sensors known from the U.S. Pat. Nos. 5,650,730 and 5,793,217, the currents that flow via the reference condenser and the measuring condenser into the analysis unit are fed via two current-voltage transformers to an adder, which processes the voltages with different signs so that a differential signal results. This differential signal is ultimately—after possible additional intermediate steps pertaining to circuit engineering—compared to a reference or threshold signal, whereby the reference signal defines a threshold, which, when reached, indicates a specific event, such as, e.g., a sufficient proximity of an object to the capacitive sensor or the presence/absence of a specific fill level.
The disadvantage to the above-described capacitive sensor is that the analysis of the current signals in the analysis unit is comparatively expensive. In particular, the presetting of a reference value, to which the difference of the current-voltage-converted currents is compared, is labor-intensive and costly in terms of circuit engineering, and in addition, is prone to frequency and amplitude fluctuations of the alternating signal. Furthermore, it has been found that the known sensors in the working frequency of 2 MHz indicated in the prior art are not suitable to be used as fill-level sensors, since they are not able to distinguish whether a medium fills a larger area of volume around the sensor or whether only a small adhesion of this medium to the sensor has remained after the medium that is to be monitored has left the area of the capacitive sensor; the fill level thus has dropped below the position of the capacitive sensor. Also, the above-described capacitance sensors are not suitable for interval operation, since, when the triggering of the current supply network is turned off, the differential current as the signal to be analyzed is also immediately brought together with the alternating signal from the alternating signal source and can no longer be used for another analysis.